The attachment of high density, microcircuit components onto substrates, such as silicon dies onto ceramic sheet, has been an important aspect of the electronics industry for many years. Generally, it is known to use a die attach paste which is deposited between the die and substrate. Typically, the die attach paste includes a filler, an adhesive and a carrier. The filler is selected to impart to the finished bonding layer desired conductive, resistive or dielectric properties. The adhesive is chosen to create a strong bond between the die and substrate. The carrier maintains all the components in a fluid, uniform mixture, which allows the paste to be applied easily to the die-substrate interface. It also has suitable volatility to migrate from between the die and substrate and leave a void-free bonding line following heat treatment of the assembly. After the paste is deposited and the die and substrate are assembled, the assembly is typically heated to fuse the adhesive and drive off the carrier. Upon cooling, the die is firmly attached to the substrate.
For the well known silver-glass pastes with organic carrier, the assembly is typically heated to as high as 450.degree. C. U.S. Pat. No. 4,933,030 discloses a silver-glass paste for attaching high density, integrated circuits at temperatures as low as 300.degree. C.
Silver-epoxy resin pastes have been developed to permit die attachment at still lower temperatures. However, epoxy resin pastes have certain drawbacks. Such pastes achieve their adhesive and strength properties by crosslinking of the epoxy resin. The crosslinking process is time consuming and therefore uneconomical. Adjustment of the formulation to crosslink faster reduces the working time during which the paste can be applied to the assembly, thereby complicating the process. Epoxy resins are two part systems consisting of resin and hardener which can require separate storage equipment and must be thoroughly mixed prior to use. Furthermore, epoxy resins generally crosslink to a rigid, unyielding state which can create excessive stress in especially large dies attached to high expansion substrate material. Thus, the die size that can be attached by an epoxy resin paste may be limited. Still further, as a crosslinked material, epoxy resins are not reworkable and therefore should not find appreciable use in the emerging, multi-chip module market.
Thermoplastic polymer resin adhesives can be used for die attachment. However, such adhesives suffer the drawback that they are solid at low temperatures and thus must be pre-formed to appropriate shape for each attachment application. Furthermore, solid adhesives are incompatible with the industry-standard, high speed production lines based on paste adhesives. In addition, the use of solid adhesives requires application of pressure to bring about a satisfactory bond. Attempts have been made to provide fluid adhesives based on thermoplastic polymer resins dissolved in solvents. Unfortunately, these systems also suffer from deficiencies, most notably, low upper limits on the amount of silver that can be incorporated while maintaining good adhesive and rheological properties, and the tendency of voids to form following solvent evolution unless excessively long devolatilization times are used.